1. Field of the Invention
The present invention relates to a substrate for an amorphous semiconductor, an amorphous semiconductor substrate comprising a substrate of this type, and a method of producing the amorphous semiconductor substrate. More specifically, the present invention relates to a substrate suitable for use to form an amorphous silicon or amorphous silicon-based semiconductor layer having excellent characteristics; an amorphous semiconductor substrate comprising a substrate of this type and an amorphous silicon or amorphous silicon-based semiconductor layer formed on the substrate; and a method of producing the amorphous semiconductor substrate.
2. Description of the Related Art
Amorphous silicon (a--Si) based semiconductors are used in a wide variety of electronic devices including light sensitive materials for electrophotography, imaging tubes, solid-state imaging devices, TFTs, solar cells, etc.
There are various known techniques to deposit an amorphous semiconductor containing Group IV elements, such as a--Si, a--SiGe, a--SiC, a--SiN, a--SiO, etc. These techniques include vapor evaporation, sputtering, and CVD such as plasma-assisted CVD and photo-assisted CVD. Of these techniques, a high frequency plasma-assisted CVD became widely used because it can provide high quality deposition for a--Si. In this technique, a film is deposited on a substrate by subjecting semiconductor source gas, for example a silane compound such as SiH.sub.4, to the decomposition using 13.56 MHz RF glow discharging. This technique was proposed first by R. C. Chittic et. at., (Journal of Electrochemical Society, Vol. 166, p. 77 (1969)). W. E. Spear et. at., (Solid State Communications, Vol. 17, p. 1193 (1975)) succeeded in pn control of electrical conductivity by impurity doping of an amorphous semiconductor. Their success attracted much attention, and induced a lot of subsequent technical developments in various applications including a solar cell (Japanese Patent Laid-Open No. 52-16990), a light sensitive material for electrophotography (Japanese Patent Laid-Open No. 54-86341), etc.
There have been various attempts to improve this technique, such as dilution of semiconductor source gas with hydrogen or Ar gas, utilization of a higher frequency than 13.56 MHz, control of substrate biasing, and utilization of electron cyclotron resonance in magnetic field.
An a--Si film deposited by a plasma-assisted CVD technique contains hydrogen in the range from a few % to several tens % (Applied Physics Letters, Vol. 30, No. 11, p. 561 (1977)). From the fact that an a--Si deposition film containing 10%-20% hydrogen shows better characteristics (photoconductivity, controllability of electric conductivity of impurity-doped material, etc.) than a--Si deposition films containing no hydrogen, it became appreciated that it is important that an a--Si film contains hydrogen.
From this view point, various techniques to supply hydrogen during film deposition processing have been investigated. For example, great positive effects can be obtained if hydrogen is supplied during the film deposition process of vapor evaporation (Journal of Applied Physics, Vol. 49, No. 12, p. 6192 (1978)).
As a variation of a sputtering technique in which a semiconductor target such as Si is sputtered by an Ar gas plasma, a reactive sputtering technique has been investigated (Solid State Communications, Vol, 20, p. 969 (1976)). In this technique, hydrogen gas is mixed with Ar gas which serves as a discharging gas, and a target is sputtered using a radio frequency ("RF") plasma of this mixed gas at a frequency of about 13.56 MHz, while inducing the reaction with deposition elements, thereby depositing an amorphous film on a substrate disposed at a location opposite to the target. This technique has succeeded in producing a fairly high quality a--Si film including a rather small amount dangling bonds.
The sputtering technique has an advantage that it does not need an expensive and hazardous semiconductor production gas or such a gas which is difficult to stock for long periods of time. Therefore, this technique does not need an expensive and large-scale protection system such as an apparatus for altering a hazardous gas into a safer form, a gas leakage alarm system, or a highly protected cylinder cabinet. It needs only very simple and inexpensive peripheral facilities.
Attempts to further improve this technique have been done. An example is a bias sputtering technique in which a substrate is biased during the sputtering process (AIP Conference Proceedings, Vol. 73, p.47 (1981), Solar Energy Materials, Vol. 8, p. 187 (1982)). Another example is to increase the discharging frequency up to much higher than 13.56 MHz so that the substrate bias may control Ar.sup.+ ions better, thereby improving the quality of a deposited a--Si film without the help of hydrogen. In addition, this technique can prevent the deposited film and the substrate from being damaged by ions. Thus, it is possible to obtain an a--Si film containing a lesser amount of hydrogen, which leads to a reduction in photo-induced degradation of photoelectric characteristics.
An a--Si film which has good photoelectric characteristics and does not show photo-induced degradation may be produced for example by reducing the concentration of hydrogen as well as the concentration of dangling bonds. To deposit such a high quality a--Si film, the deposition process should be controlled to a higher degree than in the case of conventional deposition techniques. In particular, the control of the substrate biasing is important, because it determines the amount of ions as well as energy of ions which are incident on the substrate during the deposition process, wherein the amount and energy of ions affect the quality of the deposited film.
However, in conventional plasma CVD or sputtering techniques, it is difficult to achieve sufficiently good controllability of the substrate biasing to obtain a high quality film.
For example, in a widely used RF frequency such as 13.56 MHz, ion energy of a plasma has a wide distribution. As a result, it is difficult to control the ion energy incident on the substrate, even if the substrate biasing is rigidly controlled. This problem can be solved if the discharging frequency is increased up to higher than 50 MHz so as to make the ion energy distribution sharper.
FIG. 1 schematically shows ion energy distributions of RF plasmas for a frequency of 13.56 MHz and a higher frequency than that. From FIG. 1, it can be seen that the 100 MHz plasma shows a sharp ion energy distribution, in contrast to the 13.56 MHz plasma which shows a wide ion energy distribution having two peaks.
To control substrate biasing, the substrate should have a sufficiently low resistance. A glass substrate is popular as a substrate for a--Si deposition. However, its resistance is high. Therefore, a glass substrate is unsuitable for the application of substrate biasing. Even if a very thin glass substrate having a thickness of about 0.1 mm is used and if its back surface is in contact with an electrode, self biasing will occur at the other surface which is in contact with the plasma. As a result, the DC bias application to the electrode is useless to control the substrate biasing. One known technique to avoid this problem is to apply a second RF power to the electrode that is in contact with the back surface of the substrate (Japanese Patent Laid-Open No. 63-50025). However, the control of this technique is limited to a narrow range. In particular, it is difficult to achieve a large positive magnitude of biasing.
An a--Si film can be deposited on a substrate other than an amorphous substrate such as a glass substrate, if the temperature of the substrate is maintained low enough. For example, it is possible to deposit a--Si on a metal crystal substrate such as a stainless steel substrate or an Al substrate which is held at about 250.degree. C.
However, in this technique, the migration of deposited atoms on the surface of a substrate is sacrificed, and degradation in characteristics such as photoelectric conduction often occurs.
This problem can be avoided if the substrate temperature is raised to a higher temperature, or if the reactive gas used in the plasma CVD process is diluted with hydrogen to a sufficient degree. However, in this case, the film will be often crystallized, which will bring about difficulty in producing an a--Si film.
This problem may also occur in a--Si deposition using plasma CVD. But, in particular, this problem is serious in a technique in which a--Si is deposited by bias sputtering under the conditions where the RF power frequency is higher than 50 MHz and the partial pressure of residual impurity gas other than Ar and H.sub.2 is less than 10-8 torr. In this technique, if Si is deposited on a Si single crystal wafer under optimized conditions, a Si single crystal epitaxial layer can be grown at a low temperature in the range from 250.degree. C. to 400.degree. C. This occurs because the surface of the growing film is excited by the Ar.sup.+ bombardment whereby Si atoms have a large mobility on the surface of the growing film. One technique to deposit an amorphous film keeping a high surface mobility is to use a substrate whose surface is amorphous. However, if an amorphous material such as glass is used as a substrate, another problem is the high resistance of the amorphous substrate which gives rise to difficulty in applying the bias potential to the substrate.